This invention concerns aluminum-zinc alloy plated sheet steels which have been subjected to a coating treatment. Aluminum-zinc alloy plated sheet steels are sheet steels which have been plated with an alloy which consists of 4-75% by weight of aluminum, the remainder being mainly zinc with traces of other components such as silicon, magnesium, cerium-lanthanum etc. At the present time two such types of sheet steel are being manufactured, namely low aluminum-zinc alloy plated sheet steels which are plated with an alloy consisting of 4-10% by weight of aluminum, the remainder consisting mainly of zinc with traces of magnesium or cerium-lanthanum, and high aluminum-zinc alloy plated sheet steels which are plated with an alloy consisting of 55% by weight of aluminum, 43.4% by weight of zinc and 1.6% by weight of silicon. If the thickness of the plated layer on these sheets is the same as that on the molten zinc plated sheet steels in general use the corrosion resistance is improved by a factor of some 1.5-2 times in the case of the low aluminum-zinc plated sheet steels and by a factor of some 3-6 times in the case of the high aluminum-zinc alloy plated sheet steels. Furthermore the high aluminum-zinc alloy plated sheet steels also have heat reflecting properties and excellent resistance to heat. As a result of their excellent properties these aluminum-zinc alloy plated sheet steels have found wide application as building materials in the form of roofing and walling materials etc., in civil engineering applications as guard rails, sound insulating barriers, anti-snow fencing, drainage gullies etc., as materials for automobiles, domestic appliances and industrial machinery and as base materials for painted sheet steels.
However, since the plated layer on these aluminum-zinc alloy plated sheet steels contains aluminum it is harder than the plated layers which are found on molten zinc plated sheet steels. As a result of the hardness of the plated layer the effective lubrication between the aluminum-zinc alloy plated sheet steel and rolls or pressing dies when roll forming or press molding these materials is poor: the surface is liable to damage; the plated layer may become fused onto the forming rolls or the pressing dies as a result of the heat which is generated by friction during such operations; and metal powder which has become detached from the aluminum-zinc alloy plated sheet steel may become attached to the forming rolls or the pressing dies. This powder may accumulate in the corner parts of a molded product and give rise to a grazing effect or it may cause defects such as wounds and abrasions etc. which alter the external appearance of the product.
In the past these problems have been overcome by coating the surface of the aluminum-zinc alloy plated sheet steel with a lubricant such as oil or wax using a roll coater when carrying out shaping operations in order to provide the required lubrication at the surface of the aluminum-zinc alloy plated sheet steel so that adequate slip occurs between the aluminum-zinc alloy plated sheet steel and the forming rolls or the pressing dies. However when such lubricating oils are used there is a danger that if the aluminum-zinc alloy plated sheet steel is subsequently used as a roofing material for example the operator who is doing the work may slip on the oil, and if a coating is to be applied to the aluminum-zinc plated sheet steel after the shaping process it is essential that all of the lubricating oil should be removed from the surface and this is troublesome. Furthermore when aluminum-zinc alloy plated sheet steel is shaped using a lubricating oil in the way described above the lubricating oil becomes attached to the forming rolls or the pressing dies and if at a later stage surface coated sheet steel is shaped using these same forming rolls or pressing dies the lubricating oil is transferred to the coated surfaces of these coated sheet steels which are thereby stained and any small amounts of metal powder which are produced during the shaping process become attached to the coated surface by way of the lubricating oil and so it is necessary to clean the forming rolls or pressing dies thoroughly before carrying out such operations in order to prevent the occurrence of this type of contamination and this is very troublesome. Furthermore, as in the case of the molten zinc plated sheet steels, the surfaces of aluminum-zinc alloy plated sheet steels are often subjected to a chromate treatment in order to prevent the formation of rust due to the humidity while the material is being stored. Aluminum-zinc alloy plated sheet steels have a higher resistance to rusting than molten zinc plated sheet steels, but unlike the white rust like changes which are seen to occur with molten zinc plated sheet steels the aluminum-zinc alloy plated sheet steels develop a gray to black discoloration and since this downgrades the external appearance of the material to a considerable degree they are often subjected to a chromate treatment. However if in this case the aluminum-zinc alloy plated sheet steels is treated with a water soluble lubricating oil this can lead to problems with pollution due to the dissolution of the chromate.
Hence the formation on the surface of the aluminum-zinc alloy plated sheet steels of a skin film which has lubricating properties, which provides excellent corrosion resistance and which forms a suitable undercoat for painting is desirable but as yet no surface treating agents for the formation of skin films which have these properties have been proposed. Thus conventionally surface treatments such as the chromate treatment mentioned above and non-chromate treatments such as titanic acid based treatments, polyphosphoric acid based treatments etc. in which no chromium is used have been employed. The reactive type chromate treatments necessitate chromium effluent treatment since the chromium is dissolved out in the water washing operation during the manufacturing process while with the painting dry type chromate treatments the chromium dissolves out in the degreasing process which is carried out by the end user and so in this case chromium effluent treatment is still required at this stage. This means that the non-chromate treatments are preferred, but these treatments have disadvantages in that the corrosion resistance is poor after such treatments and there are problems with the cost of the treatments. Furthermore with the aluminum-zinc alloy plated sheet steels there is no lubrication effect in shaping operations when a simple chromate skin film is formed with a chromate treatment because of the hardness of the plated layer and so it is still necessary to use a lubricating oil during such operations. Hence there is as yet no surface treatment agent which is able to provide a skin film which provides good lubrication properties, which provides excellent corrosion resistance, and which forms a good undercoat for paints on the surface of aluminum-zinc alloy plated sheet steels, and the provision of such a surface treatment agent is desirable.
The purpose of the invention is to provide coated aluminum-zinc alloy plated sheet steels for which the above-mentioned difficulties have been overcome in that they have adequate lubrication properties in shaping operations such as roll forming and press molding etc. without the need for the use of a lubricating oil; in that they have excellent corrosion resistance; and in that they can be painted directly without having to remove the skin film even if they are painted after shaping.